Coalescence Characteridtics Of Droplets In Electrical Field And The Applicability Of The Electrical Coalescencer

The dewater of crude oil is one of the most important processes of the oil field production.Electrically driven coalescence is one of the important methods to separate water from crude oils.The electrical dehydrators are challenged by the change of oil-water physical properties in the chemical flooding oil field.The electrical dehydrators do not always work.In this paper the characteristics of the coalescence,deformation and movement of the water drops in oil are studied by theoretical and experimental methods.The conditions for the electrical dehydrators to work are investigated.And the methods to improve the result of the dehydrators are proposed.The main contents and conculsions of this paper are following.The deformation of the water drops with different radius was studied by theoretical and a high-speed CCD camera system.The deformation of water drop is the result of the electric field,ineterfacial tension and the pressure difference between the inner and outer of the water droplet.The drop is not standard prolate ellipsoid after deformation,and it expands at the waist between the top and the equator.In high-viscosity liquid,the oscillation of droplet exhibits an over-damped regime,and there is no resonance frequency.The CCD camera recorded the brightness of the LEDs linked to the HV power supply which is placed behind the test cell.The phase difference between the deformation and the electric field can be calculated from the photos of the CCD.The results show that this difference increases with the increase in the frequency of the electric field,therefore electric field strength does not affect phase difference.Deformation depends on both the intensity and the frequency of the electric field in AC field.When this deformation reaches a certain level,the water drop breaks up.In order to understand the effect of the electrostatic on the coalescence of water droplets in oil,in this paper,two conducting droplets were studied numerically.The electric force of the water drop can be calculated the integral for its every parts.The simplified models for calculating the electrostatics force(point-point interaction)was revised.The effective force between drops is the force under the effective electric intensity in the AC field.The force between water drops which are ball is undepended on the frequency.The place vibration is a single degree of freedom system and it is forced bibration damped.The amplitude of the place is very little when the radius of the water drop is small,the oil viscous is high and the frequency is high in the AC field.It is same for the charged drops in AC field.The dynamic procedure for the water drops to move in electric field is very short when the radius of the water drop is small and the oil viscous is high.The frequency affects the deformation of the water drop in AC field.And the deformation affects the electric force between drops.The resistance of water drop in oil changes with the deformation too.Therefore the space changes between drops are depended on the electric field strength and the frequency in AC field.The deformation favours the drops to crash into each other.When the total volume is same,the drops with equal radius move towards each other fastest.The simplized method to calculate the time for ball drops to coalescence is proposed.The potential around the water drop,the distribution of charge on the water drop and the electric force between drops were calculated with the Finite Difference Method.When water drops contact to each other,there are different electric charges on the out sides of the drops.Little charge would occur on the inner part of the drops.In this case,there are electric forces that drag the drops to be separated from each other.There is a critical electric field strength above which the two drops would not coalesce.The critical electric field strength of larger drops is lower than that of smaller drops.If the pressure of the bridge is lower than that of the drops,Water can flow into the bridge from the drops.Otherwise,water flows from the bridge back to the drops and the bridge breaks at last.The pressure of the water drop is affected by the deformation.The deformation can be presented by the We number.The critical We for drops to coalescence is about 39 % of the critical We for drops to break.Therefore,the larger drops are likely to fail to coalescence in the electric strength.In an AC field,the average deformation increases with electric field frequency.More the deformation is,less the inner pressure is.Therefore,the breakup We number(electric strength)decreases as the frequency increases.The smaller drops have higher inner pressure;hence the drops were easier to coalescence with smaller radius in the electric field.But there are partial coalescence between big drop and small one because of the tip effect.The partial coalescence will not happen under higher intensity.However,not all of the adjacent droplets can coalescence after contact.If the droplets can not coalescence in an AC field,the droplets would be pushed away from each other at contact,and then they are dragged back again.Their approach and the push back behavior repeats.Nevertheless,the coalescence can not happen during the cycle.The frequency of the space changing between droplets is same to the frequency of the electric field.The pressure of the bridge between the drops is static pressure during the immobile drops are contacting.The pressure is lower in the bridge than that of the adjacent area when the cone angle is less than 30°.However,the moving drops have kinetic energy.The kinetic energy converts to pressure energy in the collision.The total pressure near the bridge is higher than those of the two drops and the bridge,hence the radius of the bridge can increase while the cone angle is about 48° which is higher than 30°.Under higher electric field strength,the kinetic energy of the drops is higher,so the radius of the bridge between drops increases faster at the beginning of contact.Therefore,the high velocity of drops moving in a low viscosity continuous phase favours the drops to coalesce.The principle and the applicability of the crude oil electric dehydration are studied.The parameters affecting dehydration were studied too.The results show that the electrocoalescencers should work under certain electric field strength.If the electric field strength is higher than rational value,the electric field will prevent the drops to coalesce.The effect of the electrocoalescencers will get worse.The experimental results indicated that the exponential distribution could reflect the water drops in the crude oil which is dealed with by the electrocoalescencers.In this paper,the volume-diameter distribution was proposed which presents the distribution of the total volume with some diameter.The number of small drops is huge,but the total volume of them is not large.Therefore,the dehydration rate which meets the dewater performance can be calculated.The smallest drops which shold be removed depend on the volume-diameter distribution and the needed dehydration rate.The condition for the electrocoalescencers work reliably is to increase the diameter of most drops to the value which is able to remove in the gravitational field.And the diameter of the drops is less than that the drops break up.It is very difficult to simulate the crude oil electric dehydration.The process has very complex mechanism,a lot of influencing factor,therefore simple mathematical equations is diffcult to describe these processes.At the same time,huge computation is the limitation of this method.Hence,simplified model is proposed to describe the real process.In simplified model,the needing removed smallest drops coalesce only between same diameter and the watercut is that of the needing removed smallest drops and smaller drops.The method to calculate the number that the drops coalesce from the needing removed smallest drops to the smallest drops which are able to be removed in the gravitational field is proposed.The method to calculate the space between the drops and the time to contact each other are proposed too.The result showed that the time for drops to coalesce effectively affects by the viscosity of the crude oil,the interfacial tension,the electric field intensity and the number that the drops coalesce from the needing removed smallest drops to the smallest drops which are able to remove in the gravitational field.The field test proved that the simplified model of the crude oil electric dehydration is just effective.Measures are proposed to improve the crude oil electric dehydration,include to increase the number of devices,increase the device volume,increase temperature and to optimize the electric field,space between electrodes and demulsifying agent.